Natural rubber consists of polymer of the organic compound Isoprene, with minor impurity of other organic compounds. Rubber has approximately the following composition: Water-55-70%, Rubber-30-40%, Resins-1.5-2%, Protein-1.5-2%, Ash-0.5-1%, Sugar-1-2%. Rubber is a polymer whose basic monomer is isoprene (C5H8). In 1839, Goodyear discovered that heating the rubber and sulfur together, a process called vulcanization, gave the rubber increased strength and elasticity and reduced its sensitivity to temperature. Natural rubber, when compared to synthetic, provides slightly better properties in tensile strength, tear resistance, compression, and flex fatigue resistance. Natural rubber is a polymer whose repeating unit is isoprene. The material is obtained by coagulation of latex derived from rubber tree. The molecular weight of natural rubber is in the order of 750.000 to 900.000 grams per mole. Consequently, natural rubber compounds tend to have high tensile and tear strength. Natural rubber is polymer designation 1-4 polyisoprene, empirical formula (C5H8)N. The first step in the tire manufacturing is the mixing of raw material to form the rubber compound natural and synthetic rubber, carbon black, sulfur and other chemical and oil. Rubber is water repellent and resistant to alkalis and weak acids. The advantages of natural rubber are less buildup of heat from flexing and greater resistance to tearing when hot. Track tires made from natural rubber are more resistant to cuts and tears and are more durable than made of synthetics. The physical characteristics of natural rubber, change as the temperature change, decomposes above 200 degrees Celsius is insoluble in water, alkali, and weak acid, but soluble in benzene, chlorinated hydrocarbon, and carbon disulfide. The heart of the vulcanization effect are the bonds which create what polymer scientists call cross-linkages, connecting the rubber molecules to one another in a unified network that is, for all practical purposes, no longer composed of separate molecular units. As Farris puts it, “a tire, or any rubber object, is basically one giant molecule”. Some molecules are harder than others to dismantle, and subsequent generations of scientists have had to grapple with the near-impossibility of putting asunder what vulcanization has joined together.
Vulcanization is the process by which plastic rubber is converted into the elastic rubber or hard rubber state. The process, which is brought about by the linking of macromolecules at their reactive sites, is also known as cross-linking. Vulcanizing agents are substances that bring about the actual cross-linking process. Other properties, such as tensile strength, gas permeability, low temperature flexibility and electrical resistance, change less with the degree of vulcanization. During vulcanization the long chains of the elastomer chemically cross-link. Each cross-linking releases a quantum of energy, making it an exothermic reaction. During this process, the catalyst creates a three dimensional matrix. The energy released in the exothermal reaction is proportional to the cross-linked bonds formed and it is assumed that each bond releases the same energy. The vulcanization of the sample is found by measuring the energy released as the sample is heated from below room temperature to well above the vulcanization temperature. As it was first invented by Goodyear vulcanization used sulfur (About 8 parts by weight of sulfur mixed with 100 parts of natural rubber) at 140 degrees Celsius for about 5 hours. Vulcanization with sulfur alone is no longer used today, due to the long curing times. Vulcanization of rubber is a process by which natural rubber is enhanced by creating sulfide bridges between the molecular adjacent chains, to form a more tough and controllable material which can be manipulated to suit the requirement of a specific use. This heavily cross-linked polymer has strong forces between the chains, and is therefore an insoluble and infusible, thermosetting polymer. Rubber is an example of an elastomer type polymer, where the polymer has the ability to return to its original shape after being stretched or deformed.
Vulcanization is a chemical process by which the physical properties of natural or synthetic rubber are improved. Vulcanized rubber has higher tensile strength, increased resistance to swelling and abrasion, and is elastic over a range of temperatures. One of the disadvantages of the process is reversion of the sulfur. The reversion increases with an increase of the amount of sulfur. Raw and un-vulcanized rubbers are entangled high molecular weight visco-elastic. They are generally not very sticky, strong, brittle when cold, easily deformed when warm, and incapable of maintaining their shape after a large deformation. Raw rubbers are completely soluble in solvents and have a consistency similar to inelastic deformation being made of long polymeric chain that can move independently to each other. It is not possible to use uncured rubber to make articles with a good level of elasticity.
In the process of vulcanization, the added sulfur allows some carbon-hydrogen bonds to be broken and replaced by carbon-sulfur bonds. The cross-linked molecules create a three-dimensional network of rubber. Each cross-link is a chain of about eight sulfur atoms between two long chains of polyisoprene. Strength when rubber is vulcanized it become cross-linked in its chemical structure at the atomic level. This linking of stronger bonds makes vulcanized rubber 10 times stronger than natural rubber would be. Rigidity while vulcanized rubber is elastic meaning it will return to its original shape, it is also 10 times more rigid than normal rubber as a results of the vulcanization process. Rigidity means that vulcanized rubber is more difficult to bend out of shape in the first place, adding to its use in heavier application, such as tires. The vulcanization of natural rubber by sulfur in the presence of an organic accelerator is a complicated process. Vulcanization has been generally considered as irreversible.
Synthetic rubber is made from petroleum by the same polymerization techniques used to synthesize other polymers. Some of the commercially important addition polymers are the copolymers. These are polymers made by polymerizing a mixture of two or more monomers. An example is styrene-butadiene rubber (SBR) which is a copolymer of 1, 3 butadiene and styrene which is mixed in a 3 to 1 ratio respectively.